1. Field of the Invention
The present invention relates to an image processing technique for controlling the degree of noise reduction in image data.
2. Description of the Related Art
[Prior Art of Tone Correction]
Generally, in photo shooting under the strong sunlight such as in a midsummer day, a strong contrast occurs in the face of a person and the like, which prevents clear tone reproduction of a portion hidden in a shadow or a highlighted portion. It is also difficult to clearly reproduce tone of a person photographed against the sun and hidden in silhouette etc.
There are known tone correction techniques to improve such lopsided tone representation such as histogram equalization and a technique disclosed in U.S. Pat. No. 5,991,456. These prior techniques are to improve tone collapse by performing tone correction to expand the tone range near or in the region of target pixel values of image data to be input.
[Prior Art of Noise Reduction]
Image data contains noise such as dark current of an image sensor and shot noise. There are known techniques to reduce such noise, such as a technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-101887. In general, the prior techniques are to reduce noise by performing local smoothing so as to reduce the difference between neighboring pixels of image data to be input.
The inventor of the present invention has found the following problems of these prior arts.
With the tone correction described above, the tone of a dark portion of an image is expanded, making the image brighter. This makes amplitude of noise amplified and noise hidden in the dark portion conspicuous.
In this state, one image includes a portion at which noise is conspicuous and a portion at which noise is inconspicuous and has a kind of noise unevenness therein. The
uneven noise causes visual unnaturalness more than the even noise, degrading visual quality of an image considerably.
With the conventional noise reduction method, it is able to achieve the effect of uniformly reducing the noise, but difficult to improve the above-mentioned noise unevenness.
In other words, in order to obtain a sufficient effect of noise reduction at a portion at which noise is conspicuous, it is necessary to increase the degree of noise reduction. In this case, however, the noise reduction functions excessively even at a portion at which noise is not conspicuous, resulting in unnecessarily losing fine detailed information on an image. This leads to creating the image data which gives the impression of lacking in the amount of information.
On the contrary, through the noise reduction in accordance with a portion at which noise is not conspicuous, noise is not sufficiently reduced from a portion at which noise is conspicuous, leaving unnatural noise unevenness.